Field of the Invention:
The invention relates to a data carrier with at least one coil for the contactless reception of amplitude-modulated signals, a rectifier circuit connected downstream of the coil and a circuit configuration for processing and/or storing data.
Such a data carrier is presently on the market mainly in the form of a chip card and is known from German Published, Non-Prosecuted Patent Application DE 196 34 134 A1, corresponding to U.S. application Ser. No. 09/256,283, filed Feb. 23, 1999. In the case of the method described therein for transmitting data between a terminal and a portable data carrier through a wireless electromagnetic transmission link, a 100% ASK modulation of the carrier signal takes place, as is generally customary in the case of such present-day contactless transmissions. Although that switching on/off of the carrier signal is relatively easy to demodulate in the data carrier, it has the disadvantage that no clock signal is available during a blanking interval.
However, in standardizing bodies for a contactless chip card (ISO 14443), the current standpoint is to use not only ON-OFF keying (OOK) but also amplitude-shift keying (ASK) with a degree of modulation of 5 to 15% for the data transmission from a writing/reading device to a card or generally to a data carrier.
However, such modulation is difficult to demodulate since the distance between the writing/reading device and the data carrier can change greatly and, as a result, the amplitude of the received signal is subject to fluctuations which are superposed on the modulation and falsify it. In addition, the circuits in the data carrier have a greatly fluctuating power consumption, which likewise has retroactive effects on the modulation.
The problems referred to are particularly critical in the case of passive data carriers, which have no power supply of their own and obtain their operating energy from the signal being received.